A number of types of apparatus and systems for recycling asphaltic concrete have been proposed. Included are those described in my U.S. Pat. Nos. 3,849,040, 3,999,743, and patents to others such as U.S. Pat. Nos. 4,075,710, 4,147,436 and 4,165,184. In these state of the art systems, the recycle material is introduced into an elongated rotating drum at the same end at which a burner introduces hot gases of combustion for heating the material, or at some point intermediate the ends of the drum. Thus, these systems have in common the more conventional method of gradually advancing the recycle material along the drum length in the same direction as the burner gases are directed. There is a disadvantage in using such an apparatus in heating asphaltic concrete which contain substantial amounts of asphalt "fines" which are easily burned when exposed to the high temperatures in the drum unless shielding or other equivalent means is incorporated to prevent overexposure of these small particles to the substantial heat. Alternatively, the recycle material is usually added downstream from the burner in a cooler drum zone. Although this latter method may be effective in preventing asphalt burning, it also substantially reduces the exposure of the asphaltic concrete to heating which may necessitate increased residence time within the drum or require super heating of the aggregate material at the hot end of the drum, in order to achieve the desired final product temperatures.
In U.S. Pat. No. 4,130,364 there is disclosed an apparatus for recycling asphaltic concrete by introducing the composition at the drum end opposite that at which the flame from the burner is introduced and advancing it toward the burner. The apparatus also requires a special flight installment so that a veil of composition is dropped through the center of the drum in the drum portion furthest from the burner, whereas in the drum portion nearest the drum burner, the composition is dropped to the side of center. In this apparatus, as well those previously noted, a common drawback is in venting the exhaust gases from the drum to atmosphere, or requiring treatment of such exhaust gases in conventional dust filtration and collection apparatus in an attempt to meet environmental pollution requirements. However, where the recycle asphaltic concrete is heated to temperatures above about 225.degree. F., and commonly above 250.degree. F. and higher, substantial amounts of volatile hydrocarbon gases and other asphaltic volatiles are given off by the hot asphalt, and which cannot be removed by such conventional exhaust gas treatment.
In my U.S. patent applications Ser. Nos. 871,351 filed Jan. 23, 1978, 906,734 filed May 17, 1978, and 139,709 filed April 14, 1980 there are disclosed improved methods of heating asphaltic concrete during recycling which burns these hydrocarbon volatiles within the drum prior to being vented to atmosphere. Moreover, in such apparatus the exhaust gas products are vented to atmosphere substantially only through the same port into which the burner directs flame and hot gases of combustion. Thus, these apparatus incorporate a substantially "closed end" drum system in the recycling process. Particularly in the latter patent application, where the recycle composition is introduced at the drum end opposite that at which the burner introduces flame, the system lends itself to apparatus scale up to handle continuous composition recycling quantities of say, 50 tons a hour, and up. Although such an apparatus offers the advantages of substantially increased production rates, while at the same time avoiding composition burning and degradation, but which burns the volatile asphaltic hydrocarbons prior to venting the exhaust gases to atmosphere, it is found that much heat is lost in the process. For example, the temperature of the exhaust gas vented to atmosphere in such an apparatus may be as high as 800.degree. F., or more, which represents a substantial loss of energy. It is to the reduction of such energy loss, as well as the overall improvement of recycling processing that the present invention is directed.